Disc brake calipers commonly use hydraulically-actuated pistons to press against the disc brakes. The piston can slide in a forward direction within a cylinder under the force of hydraulic fluid, with an elastomeric seal preventing leakage of the fluid. Where the braking system is operated continuously for long periods of time, as in braking an airplane or race car, severe heating becomes an important problem. The heat can boil away the hydraulic fluid and can damage the elastomeric seal. A piston assembly that minimized the temperature rise of the seal and hydraulic fluid, would be of considerable value braking systems of conventional overall design for heavy-duty applications.